Breaking taps and shank dia being turned down to shorten thread length
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  1. #1
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    Default Breaking taps and shank dia being turned down to shorten thread length

    Hello, my question specifically deals with perception. I have a situation where someone broke a tap using it on a CNC mill. All other reasons and advice aside, this is dealing with what has polarized many people here. The tap used was a better quality 6-32 tap that has a thread length of roughly 0.440" and the 0.140" shank is turned down to 0.095" prior to the start of the thread.

    For clarity and if anyone wants to look one up it was an OSG 6-32 split point tap and the MFG# is 1212400

    Now, for most sizes these taps and many other brands in this style have held up very well. This time I am being pushed to stop getting these "weakened" taps and opt to get taps where the shaft has not been turned down: such as the Hertel taps that have a full 0.140" diameter shank all the way up to the threads and the thread length is longer, at around 0.850".

    Sorry for the long rambling but all things being equal, is a tap like the OSG weaker and more likely to break when tapping using a CNC mill or I suppose even hand tapping?

    I also do not like the idea that I have been told to, when possible, not get the "turned down" taps when a regular shank dia. tap is available. This seems wrong to me.

    I fully realize that many, many more things are at play here. Material, hardness, hole type, speeds, feeds, etc. and the reasons for breakage can be complicated. I want to address the idea that a tap should not be chosen simply because of a singular feature (the turned down section) on the basis that it is weaker and will break faster than an identical tap with a full diameter in the same situation.

    My question is simply this: Do I always choose a full shank diameter tap versus one that has been turned down because it is stronger and will not break as quickly?

    I hope my rambling makes sense....Thank you for taking the time.

  2. #2
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    I would say no, they are not weaker. I happen to like OSG taps

    that is not why they are breaking

  3. #3
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    To me the main reason for using reduced shank taps is when holes need to be tapped slightly deeper than the length of the thread cutting section. For applications other than that I tend to use the full diameter shank taps.

    I don't think there's an inherent weakness with reduced shank taps. I've seen either style break when pushed beyond their limits as far as speed and longevity are concerned. Most major brand taps I've purchased recently seem to have a service life of between 1,000 and 1,500 holes before they show signs of wear. I'm sure you can push some of the higher end ones farther, but I tend to replace them at that point just because I hate breaking them, and hate having to remove the broken pieces even more.

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    No test data to go by but I would think that as long as the shank wasn't turned down smaller than the root diameter of the thread then the strength of the two types would be pretty much the same. I know there is a significan stress riser at the transition point where the threads run into the full-diameter body. Every tap I've broken when power tapping has been at that point. Without that transition to a larger body there would not be nearly as large as a stress riser there.

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    An aside: avoid 6-32 wherever possible. Has the largest major/minor ratio of any tap size, is the weakest fastener and the weakest tap.

    When tapping 6-32 in tough stuff or extended depth, upsize the tap drill size as much as possible.


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